Voltage-controlled semiconductor switches, such as MOSFET's and IGBT's, are almost exclusively used in electronic actuators. Such electronic actuators are used, for example, as d.c. chopper controllers, pulse-controlled inverters, and the like. Such as circuit normally has a current path, which includes a load inductor, a free-wheeling diode, and a semiconductor switch, in particular in the form of a field-effect transistor. In addition, the line connections between the components have inherent inductances, which are parasitic and affect the switching performance in the current path.
In order to bring the field-effect transistor from a conducting state into a blocking state or from a blocking state into a conductive state, its gate-source capacitor must be discharged or charged. Therefore, a comparatively high recharging current (for instance, between 0.5 and 4 A) is necessary for a transition that is as rapid as possible (customary recharging times are only several hundred nanoseconds). These recharging currents are normally provided by a driver circuit. In particular, such a driver circuit has a voltage source, which charges or discharges the gate-source capacitor through a resistor, the resistor allowing the magnitude of the gate current to be changed and therefore allowing the switching rate of the power semiconductor to be controlled.
In addition, a diode circuit may be provided with a diode and a resistor, the diode being connected in parallel to the resistor, in order to allow the gate-source capacitor to discharge more rapidly during a circuit-breaking operation. The setting of the charging and discharging rates of the gate-source capacitor is important, since when the field-effect transistor is abruptly switched off due to the inductances in the current path, the voltage across the drain and source of the field-effect transistor increases beyond the provided supply voltage. Since the field-effect transistor only has a certain dielectric strength, by which a maximum drain-source voltage is predetermined, the rate of change of the charging or discharging current for the gate-source capacitor must be specified. A lower rate of change of the gate capacitor can reduce unwanted effects, such as the excess voltage between the drain and source terminals due to parasitic inductances, disruptive emissions, and reverse diode currents, but then the switching losses, i.e. the power in the field-effect transistor converted into heat, increase considerably. Fixing the charging or discharging rate of the gate-source capacitor only allows a compromise to be obtained between, on one hand, switching losses and, the other hand, excess voltages and disruptive emissions.